In Japan, since ancient times, the act of cleaning places of worship has been considered to be a very valuable form of self-purification. It is customary to clean the Shrine and the Shrine area each day before meditation and thoroughly twice a month.
By offering one's service without expecting any reward, out of one's gratitude to God, the individual has a chance to break through, however so slight, the hard shell of his own limited ego. The offering of one's service is, of course, not limited to holy places. The conscious effort to save natural resources for the benefit of the coming generations or the cleaning of public utilities, for example, with the desire to promote harmony within the community, leads to the same end.
As human beings, however, the more earnestly we work, the more rewards we expect- this is human nature, and the more we attach ourselves to the fruits of our actions, the more difficult it becomes to break through the hard shell of our limited existence. So, when one is engaged in an action such as cleaning in the Shrine, with an attitude of gratitude and respect to God, it is a time and a place where thoughts of reward or results can be more easily transcended.
Selfless action, especially if performed in a place of worship, is a very powerful aid in helping us to accept and to subsequently overcome our karma.